This invention relates to a developing apparatus for developing an electrostatic latent image, and more particularly to a developing device for developing an electrostatic latent image produced on a surface of a photosensitive layer by means of a magnetic developing agent.
In recent years, developments in an electrostatic photographic apparatus have largely been brought about by devices provided with a magnetic brush constituted of a one component magnetic toner used as a magnetic developing agent.
A typical electrostatic photographic apparatus is constructed, as shown in FIG. 1, by arranging a developing roll 16 which includes a magnet roll 12 having a plurality of magnetic poles so as to face an electrostatic latent image-forming plane (a developed plane) 10 of a photosensitive body and a cylindrical sleeve 14 surrounding the magnet roll 12.
The magnet roll 12 and cylindrical sleeve 14 are rotated in the opposite directions as indicated by the arrows given in FIG. 1, or in the same direction as shown in FIGS. 2 and 3, or the magnetic roll 12 alone is rotated, thereby providing a hollow cylindrical magnetic brush 20 formed of magnetic toner 18. Further, the cylindrical magnetic brush 20 is controlled to have a uniform wall thickness by a doctor blade 22. In other words, the conventional electrostatic photographic apparatus is the type in which the magnetic brush 20 is made to frictionally slide over the electrostatic latent image-forming plane 10 of the photosensitive body.
The above-mentioned prior art development process indeed assures an appreciably satisfactory picture quality. However, the process although retaining certain merits still has the following disadvantages. The development process of FIG. 1 indeed assures picture quality which is well balanced as a whole. However, the magnetic toner 18 which is deposited or is going to be deposited on the electrostatic latent image-forming plane 10 of the photosensitive body is rotated as a result of the rotation of a series of magnetic poles. Consequently, it is known that the solid image section of a developed latent image is accompanied, as shown in FIG. 4, at the rear or front end (as viewed in the traveling direction X of a copy sheet) with an unsatisfactory pattern commonly referred to as "tailing". The tailing appears, as shown by the reference numeral 24 of FIG. 4, in the form of, for example, a comet tail at the rear end of the solid image section 28 on a copy sheet 26.
Another drawback of the conventional development process is that an impression on the copy sheet 26 is accompanied with the phenomenon in which lines 30 of a developed latent image parallel with the traveling direction X of the copy sheet 26 are clearly impressed, whereas lines 32 of the developed latent image extending at right angles to the traveling direction X are indicated indistinctly. When, therefore, a graphed or pencil-written original impression is copied, an extremely unsatisfactory pattern arises. The reason why the aforementioned lines 32 extending across the traveling direction X of the copy sheet 26 become unclear is that the magnetic brush 20 formed of the magnetic toner 18 on the outer peripheral wall of the cylindrical sleeve 14 is rotated only in one direction, causing that magnetic toner 18 of lower adhesivity to the photosensitive body which forms the crosswise extending lines 32 to be partly scraped off by the rotation.
With the prior art development process of FIGS. 2 and 3, the hollow cylindrical sleeve 14 and magnet roll 12 are rotated in such a direction as offsets the traveling speed of the magnetic toner 18. As a result, the traveling speed of the magnetic toner 18 is retarded to reduce the force with which the impressed lines 32 are scratched off, improving the reproducibility (as counted among the various drawbacks accompanying the prior art development process of FIG. 1) of indistinct crosswise extending lines 32. However, the picture quality of an impression apprearing on the copy sheet 26 is rendered unsatisfactory due to the slow traveling speed of the magnetic toner 18. Where an insulative toner is used for the developing agent, specifically, the copy sheet cannot be put to practical use. Further, the problem of the aforementioned tailing phenomenon which accompanies the developed latent image due to the rotation of the magnetic roll 12 still remains unresolved.
Another development process has been proposed which carries out development by two developing rolls 16. This proposed development process, however, only ends with improvement in the density of an impression produced on a copy sheet, or in the reproducibility of the density of an original impression. To date, no developing apparatus has been proposed which can resolve all the drawbacks encountered in the past.